Analysis and countermeasures of several common film defects

whitish

During the construction of quick-drying coatings, whitening often occurs, and the coating film loses its gloss and cannot be restored. These coatings are mainly nitro lacquer, perchlorethylene paint, phosphating primer, thermoplastic acrylic enamel and so on. The reasons and measures are as follows.

(1) The humidity of the working environment is too high, and a large amount of heat is taken away when the solvent evaporates rapidly, causing the surface temperature of the local wet film to drop below the "dew point", causing water vapor to condense and penetrate into the wet film, resulting in an emulsified white film, and voids are formed after the water finally evaporates and is replaced by air, eventually forming a porous matte white film that cannot be recovered. The relative humidity of the control environment is below 70%, and the temperature is around 20°C.

(2) The volatility of the paint solvent is too fast. Choose a diluent with appropriate volatility, or add some slow-volatile strong solvents to the original thinner to adjust the volatility of the thinner, such as dipropylcetone, cyclohexidine, butyl acetate, butanol, ethylene glycol with acetate, etc. .

(3) The surface temperature of the object to be coated is too low. The workpiece can be heated to 1o ℃ higher than the ambient temperature.

(4) The wet film contains moisture. Check the paint, whether there is water in the thinner, whether the oil-moisture regulator of the Air Compressor is invalid, and whether the oil and water of the Air Compressor and the oil-moisture regulator are regularly discharged.

(5) Improper selection of diluent. The true solvent in the thinner volatilizes too quickly, and the non-solvent volatilizes slowly. When the wet film is dried, the resin is insoluble in the non-solvent and precipitates white with the volatilization of the solvent.

drawing

When spraying, the paint is sprayed out in a filamentous form, which affects the appearance of the coating film. This problem is prone to occur with acrylic paint, vinyl chloride paint, chlorinated rubber paint, epoxy resin paint, etc.

(1) The construction viscosity of the paint is too high, the atomization is not good, and it is easy to produce filamentous spray. Adjust construction viscosity

(2) The dissolving power of the thinner is not enough, and the diluting property is too poor. Add a strong solvent to enhance the solubility of the thinner.

(3) The molecular weight of the paint resin is too high, and the viscosity is very high under normal construction solids. Increase the amount of thinner.

Shrinkage cavity

When the difference between the surface tension of the coating and the surface tension of the substrate is large, it is easy to cause poor wettability of the coating to the substrate, and the angle of the contact angle becomes larger, so that the coating tends to keep picking up and expose the coated surface. The aperture size is generally 0.1~2mm. Especially varnishes and paints with less pigments are more prone to this defect and are not easy to repair.

(1) The cause of the paint itself, such as too much surface tension of the paint, poor fluidity, resulting in poor wetting and poor air bubble release.

(2) When there is oil or water on the surface of the substrate, it will cause poor wetting of the coating film.

(3) The smooth old paint film is also prone to poor wetting of the paint film. Give proper sanding.

(4) Oil stains are mixed into the paint, such as oil and water from compressed air mixed into the paint.

(5) The base coating contains silicone oil additives or a plastic surface with a release agent, and the surface tension of the substrate is too low, resulting in poor wetting. The low polarity surface layer is removed by sanding.

(6) The air in the drying room is dirty, and it is easy to cause shrinkage cavities.

(7) The wet film is not polluted by water, oil and other sprays.

Bubbles and pores

The pores are the pores left when the air bubbles trapped in the coating film forcefully break through the coating film and escape during the drying process of the coating film. Bubbles that make the coating film bulge without being broken are called bubbles. The air bubbles can be the air bubbles formed when the paint is stirred, or the solvent bubbles formed by the rapid volatilization of the solvent during drying.

(1) The solvent evaporates too quickly. Adjust the solvent volatility and extend the flash-off time.

(2) The base coat is not dry and contains solvent. Allow the undercoat to dry before reapplying.

(3) The viscosity of the paint is too high. Dilute according to construction specifications.

(4) The paint is stirred too vigorously, or sprayed immediately after stirring. You can add some defoamer or alcohol solvent, and let it stand for 30 minutes after stirring.

(5) When brushing, the brush moves too fast or the bristles are too hard.

(6) The temperature rises sharply when thick coating or water-based paint is baked. Extend the flash-off time and heating process.

(7) Moisture remains on the surface to be coated. Add a surface drying process, or improve the drying effect.

pinhole

Pinholes refer to small holes on the surface of the coating that are pierced by needles, similar to leather pores, with a pore diameter of about 100 µm.

(1) The varnish is poorly refined and contains impurities.

(2) The solvent in the paint evaporates quickly, and the amount is large.

(3) Pigment dispersion is poor.

(4) The coating has high surface tension, high viscosity, poor fluidity and poor air bubble release.

(5) The mutual solubility of the paint base is poor. With the decrease of the ambient temperature, the local precipitation or the increase of the viscosity will easily cause particles and pinholes. For example, asphalt paint is most likely to produce this defect.

(6) The paint is stirred for a long time, forming countless fine air bubbles.

(7) The object to be coated is not cooled, so that the wet film solvent evaporates rapidly.

(8) The coating film is sprayed too thickly and the surface dries too fast.

(9) The working environment is too high, or moisture is brought into the coating film during spraying.

(1o) Insufficient flash-off. The oven heats up too quickly.

Blossom

Flooding is an appearance defect in which the color tone of the paint surface is inconsistent, and spots and streaks appear.

(1) The pigments in the paint are unevenly dispersed, or the color paste is not fully mixed when toning. Such as gray, light blue, green paint, etc. are more prone to this problem.

(2) Poor solubility of thinner, or improper viscosity, is not conducive to the uniform dispersion of pigments and colorants.

(3) Convection occurs in the upper and lower layers of the thick coating film. If the density difference between the two pigments is large, it is easy to bloom and form small hexagonal patterns.